The present invention relates to a method for producing sintered cadmium negative electrode for use in nickel-cadmium storage batteries.
Cadmium negative electrodes for use in nickel-cadmium storage batteries known heretofore are classified into sintered negative electrodes and non-sintered negative electrodes. A sintered negative electrode is produced by filling a nickel sintered substrate, which is prepared by sintering nickel powder, with a negative active material made of cadmium oxide or cadmium hydroxide. On the other hand, a non-sintered negative electrode is produced by first preparing a paste by kneading a negative active material comprising cadmium oxide or cadmium hydroxide with synthetic fibers, glue material (binder), etc., and then coating and adhering the resulting paste to an electrically conductive core body (substrate) such as a punching metal and the like.
In the negative electrodes above, a sintered negative electrode exhibits high reactivity, high charge and discharge efficiency, and excellent properties in absorbing gaseous oxygen, because the negative active material is brought into direct contact with the nickel sintered substrate having excellent electric conductivity. The sintered negative electrode of this type is produced by repeating, for several times, a sequence of operation comprising immersing the nickel sintered substrate in an aqueous cadmium nitrate solution, drying, and converting the resulting product into cadmium hydroxide in an alkaline aqueous solution. In this manner, a sintered substrate filled with a predetermined quantity of negative active material (cadmium hydroxide) can be obtained.
However, the as-prepared sintered substrate filled with a predetermined quantity of cadmium hydroxide not only is still low in activity of the active material, but also contains impurities such as nitrate groups in the electrode plate, which badly influences the cell characteristics. Hence, the cadmium negative electrode filled with the active material requires chemical conversion treatment. Such a chemical conversion treatment comprises, in general, charging and discharging the cadmium negative electrode filled with the active material in an alkaline aqueous solution for one to several times. However, a chemical conversion treatment by charging and discharging led to a problem of decreasing the production efficiency, because it increased not only the process steps, but also the time duration of treatment.
Accordingly, a method for removing impurities such as nitrate groups by applying heat treatment to the cadmium negative electrode after filling it with an active material was proposed in, for instance, JP-A-61-85772 and JP-A-62-115662. Since the method proposed in JP-A-61-85772 and JP-A-62-115662 simply requires the cadmium negative electrode filled with the active material to be heat treated at a temperature of 200xc2x0 C. or higher under an inert gas atmosphere, it enables improving the productivity of the cadmium negative electrode of this type; i.e., it allows removal of the impurity in less process steps and in a shorter period of time, and is thereby suitable for continuous treatment.
In the cadmium negative electrode of the type above, however, positive electrode control (i.e., controlling the capacity of the positive active material lower than the capacity of the negative active material) should be maintained. Accordingly, a pre-charging step for imparting discharge reserve to the cadmium negative electrode after filling it with the active material is provided. Hence, even in the method proposed in JP-A-61-85772 and JP-A-62-115662 above, pre-charging for imparting the discharge reserve must be carried out after the heat treatment.
In case the cadmium negative electrode is immersed in an alkaline aqueous solution for pre-charging, cadmium oxide generated by heat treatment undergoes hydration to generate cadmium hydroxide. However, since cadmium hydroxide thus generates is low in electrochemical activity, there occurred a problem that the pre-charging time must be taken longer to achieve the necessary discharge reserve by pre-charging. Since a longer pre-charging time increases the amount of charge, this led to a problem of making low-cost production unfeasible. Furthermore, since the pre-charging time is also elongated, there occurred another problem that the production efficiency is lowered due to the increase in production time. Moreover, in case the pre-charging time is shortened in order to reduce the production time, it resulted in insufficient amount of discharge reserve, and caused a problem of poor cycle characteristics.
In the light of such circumstances, in JP-A-11-273669 was proposed a method comprising adding polyvinyl pyrrolidone of relatively low degree of polymerization, which is excellent in charge-discharge characteristics, into cadmium oxide active material, such that the charge acceptance of the cadmium negative electrode should be improved. However, this method only improves the inferior charge acceptance of the polyvinyl pyrrolidone of relatively low degree of polymerization to a level well comparable to that of a known cadmium negative electrode. Further, there was found another problem that, although polyvinyl pyrrolidone was effective on improving discharge characteristics, no effect was discernible on improving the charge characteristics.
On the other hand, in Japanese Patent No. 2567672 is proposed a method comprising firing a cadmium negative electrode filled with an active material at a temperature of 200xc2x0 C. or higher to convert the active material into cadmium oxide, and then adding a polysaccharide or a derivative thereof having a polymerization degree of 320 or higher. In case of the cadmium negative electrode proposed in Japanese Patent No. 2567672, the active material is added in the state of cadmium oxide having a smaller volume. Thus, since a larger amount of polysaccharide can be added to the electrode plate as compared to a case of adding in the state of cadmium hydroxide, the degradation in characteristics can be suppressed at the charge-discharge cycles; furthermore, by thus performing chemical conversion by charging and discharging after the addition, the utilization factor can be elevated as compared with an electrode plate to which the polysaccharide is added after the chemical conversion. However, the use of a polysaccharide with a polymerization degree of 320 or higher resulted in the formation of a stubborn polymer film, and this film functioned as a resistance on charging and discharging. Thus, this led to a problem of reducing, in particular, the operation voltage on discharge.
In the light of such circumstances, the invention has been made to overcome the problems enumerated above, and an object thereof is to provide a method for producing a cadmium negative electrode having excellent cycle characteristics, without impairing the production efficiency even in case the impurities incorporated during filling the active material should be removed by heat treatment.
In order to achieve the object above, the method for producing sintered cadmium negative electrode according to the invention comprises: an active material filling step comprising filling the nickel sintered substrate with an active material based on cadmium hydroxide to obtain an active material filled electrode plate; a heating step comprising heating the active material filled electrode plate to change at least a part of the thus filled active material based on cadmium hydroxide into cadmium oxide; a polyvinyl alcohol adding step comprising adding polyvinyl alcohol into the active material filled electrode plate through the heating step; and a hydration step comprising hydrating the active material filled electrode plate added with polyvinyl alcohol (i.e., a step comprising immersing the electrode plate in an alkaline solution to convert cadmium oxide into cadmium hydroxide).
In case the electrode plate filled with the active material is subjected to heat treatment, most of the filled cadmium hydroxide (Cd(OH)2) is converted into cadmium oxide, and by immersion in an aqueous alkaline solution in the subsequent hydration step, it is hydrated and re-converted into cadmium hydroxide. The cadmium hydroxide thus generated by hydration is xcex2-type cadmium hydroxide having a smaller surface area. However, in case polyvinyl alcohol (PVA) is added to the heat-treated electrode plate prior to the hydration, polyvinyl alcohol reacts with the active material (cadmium hydroxide) on hydration as to generate xcex3-type cadmium hydroxide having an acicular crystal structure and a larger surface area. Since cadmium hydroxide having a larger surface area results in an improved charge acceptance, the quantity of charged electricity can be reduced at pre-charging. Accordingly, this enables cadmium negative electrode with lower quantity of charged electricity and having excellent cycle characteristics.
Since the temperature at which cadmium hydroxide converts into cadmium oxide is ca. 180xc2x0 C., the temperature for heat treatment should be set to 180xc2x0 C. or higher.
Furthermore, in case the amount for converting cadmium hydroxide into cadmium oxide is less than 70% by mass with respect to the total mass of the active material as reduced to cadmium hydroxide, impurities such as nitrate groups in the active material remains insufficiently decomposed. Such impurities negatively influences when assembled into a cell as to increase self discharge. Hence, the amount converted into cadmium oxide must account for 70% by mass or higher with respect to the mass of total active material as reduced to cadmium hydroxide.
In case the amount of adding polyvinyl alcohol (PVA) is too small, polyvinyl alcohol tends to insufficiently react with the active material, and this results in an insufficient formation of xcex3-type cadmium hydroxide having an acicular crystal structure and a larger surface area.
On the other hand, polyvinyl alcohol added in excess inhibits the charge-discharge reaction. Accordingly, the amount of adding polyvinyl alcohol is preferably confined as such that it may fall in a range of from 0.03 to 10% by mass with respect to the mass of the total active material as reduced to cadmium hydroxide.
Furthermore, in case the polymerization degree of polyvinyl alcohol (PVA) added prior to hydration should exceed 2000, the coating film of polyvinyl alcohol (PVA) that is formed on the surface of the negative electrode becomes too stubborn as to inhibit the charge-discharge reaction. Thus, preferably, polyvinyl alcohol (PVA) having a polymerization degree of 2000 or lower is added into the cadmium negative electrode prior to hydration.